Large scale commercial manufacturing and refining processes typically use a process controller to control the operation of one or more process control devices such as control valves which, in turn, control one or more process variables, such as fluid flow, temperature, or pressure within the process. Generally, a process control valve has an actuator controlled by a positioner that moves an associated control element, such as a valve plug, a damper, or some other alterable opening member, in response to a control signal generated by the process controller. The control element of a control valve may, for example, move in response to changing fluid pressure on a spring-biased diaphragm or a piston head or in response to the rotation of a shaft, each of which may be caused by a change in the control signal. In one standard valve mechanism, a control signal with a magnitude varying in the range of 4 to 20 mA (milliamperes) causes a positioner to alter the amount of fluid and thus, the fluid pressure, within a pressure chamber in proportion to the magnitude of the control signal. Changing fluid pressure in the pressure chamber causes a diaphragm to move against a bias spring which, in turn, causes movement of a valve plug coupled to the diaphragm.
Process control devices usually develop or produce a feedback signal indicative of the response of the device to the control signal and provide this feedback signal (or response indication) to the process control device for use in controlling a process. For example, control valves typically produce a feedback signal indicative of the position (e.g., travel) of a valve plug or other moveable valve member.
Even though control valves may use these feedback signals to perform functions within a process control loop, it has been discovered that poor control loop performance may still be caused by poor operating conditions at the control valve. In many cases, problems associated with the individual process control devices cannot be tuned out of the control loop by the process controller and, as a result, the poorly performing control loops are placed in manual or are detuned to the point where they are effectively in manual. In this case, the processes associated with these control loops require constant supervision by one or more experienced human operators, which is undesirable.
Poor control loop performance can usually be overcome by monitoring the operational condition or the "health" of the process control devices connected within a process loop, and repairing or replacing the poorly performing process control devices. The health of a process control device can be determined by measuring one or more parameters associated with the process control device and determining if the one or more parameters is outside of an acceptable range.
One process control device parameter that may be used to determine, and that is indicative of, the health of a process control device is dead band. Generally speaking, in process instrumentation, dead band is the range through which an input signal may be varied, upon reversal of direction, without initiating an observable change in an output signal. Dead band, which may be caused by the physical play between mechanically interconnected components, friction, and/or other known physical phenomena, is best observed when a control signal causes a reversal in the direction of movement of a moveable element of a process control device. During this reversal, the control signal undergoes a discrete amount of change (dead band) before the moveable element of the process control device actually exhibits movement in the new direction. Put another way, the difference between the value of the control signal at which movement of the process control device element in a first direction last occurred and the value of the control signal at which the movement of the process control device element first occurs in a second and different direction is a measure of the dead band of the process control device.
Referring to FIG. 1, rough estimates of the dead band have been obtained by applying a blocked sinusoidal signal to a process control device. The blocked sinusoidal signal includes periods of alternating steps of equal magnitude that increase in amplitude from period to period, such as 1%, 2%, 5%, and so on. Once movement of the valve element or the process variable occurs following a reversal of direction, the amplitude of the step (doubled) provides an upper bound on the dead band. The lower bound is provided by the amplitude of the steps in the preceding period.
Other device parameters that may be used to determine the health of a process control device are dead time, response time, gain, and overshoot. Dead time is associated with, and may be considered to be a measurement of the amount of time it takes the process control device to actually begin moving a moveable element in response to a change in a control signal. Response time is the amount of time it takes the moveable element of a process control device to reach a certain percentage, for example, 63 percent, of its final value in response to a change in a control signal. The gain of a process control device is indicative of the amount of amplification caused by a change in the control signal. The gain may be expressed as the ratio of relative change in valve travel to relative change in the control signal. The overshoot of a process control device indicates how much a valve travels beyond its eventual steady-state position.
If the dead band, dead time, response time, or other process control device parameter(s) of a process control device increase a significant amount over their nominal values, it may be necessary to repair or replace the process control device to establish adequate control within the process control loop. However, it is not usually very easy to measure process control device parameters, such as dead band, dead time, response time, gain, and overshoot to monitor the health of functioning process control devices when those devices are connected on-line within a control loop.
In the past, operators have had to remove a process control device from a control loop to bench test the device or, alternatively, control loops have been provided with bypass valves and redundant process control devices to make it possible to bypass a particular process control device to thereby test that device while the process is operating. Otherwise, operators have had to introduce significant perturbations into the process operation or wait until a process is halted or is undergoing a scheduled shut-down to test the individual process control devices within the process. Each of these options is time consuming, expensive, and potentially disruptive to the process, while still only providing intermittent measurement of the parameters of the individual process control devices required to determine the operating condition of those control devices.